1. Field of the Invention
The present invention relates to a cord switch suitable as an object detection sensor for detecting an object or a person being stuck in or close to being stuck in an automatic door or the like.
2. Related Art
For example, some vehicles are provided with a sensor attached to an edge of power sliding door, a power rear door or the like such that it detects a state that an object or a person is caught or close to being caught between the door and the vehicle body so as to open the door for safety.
Such a sensor includes a cord switch, and a cord switch of contact type is known in which ON/OFF operation is conducted in response to contact between plural electrode wires caused by pressure changes occurring when an object contacts a power window or door (e.g., WO 97/21235).
FIG. 4 shows the structure of a conventional cord switch disclosed in WO 97/21235. This cord switch 40 has the structure that four electrode wires 42 are circumferentially disposed at predetermined intervals on the inner periphery of a hollow insulation 41 with a restoring force, and the electrode wires 42 are each helically arranged in the longitudinal direction.
The hollow insulation 41 is formed of a material, e.g., a restoring rubber or restoring plastic, that holds the electrode wires 42 not to electrically contact each other when external force is not in action, is easily deformed by the external force to allow the electrode wires 42 to electrically contact each other, and is restored immediately when the external force is removed.
The electrode wire 42 is composed such that a conductive rubber layer or a conductive plastic layer 44 is formed on the outer periphery of a metal layer 43 formed of one or multiple stranded wires.
Further, a combined contact/contactless cord switch is known in which outer electrodes are disposed on the periphery of the contact-type cord switch 40 such that a capacitive contactless detection function is added as the contact detection function (e.g., JP-A-2007-123202).
FIG. 5 shows the structure of the combined contact/contactless cord switch in JP-A 2007-123202. The cord switch 50 is composed such that outer electrodes 51 are formed by laterally winding thin copper wires on the outer periphery of the conventional cord switch 40 in FIG. 4, and a sheath 55 covers the outer electrodes 51 for protecting the outer electrodes 51.
When an object approaches the cord switch 50, capacitance formed between inner electrode wires 52 and the outer electrodes 51 varies due to floating capacitance formed between the approaching object and the outer electrodes 51. By reading the variation of the capacitance, contactless detection of the object can be conducted.
However, since the conventional combined contact/contactless cord switch 50 may be exposed to high-temperature environment in case of a vehicle, an outdoor use or the like, a problem may arise that ON-resistance in turning on the cord switch 50 increases to deteriorate the detection sensitivity when the inner electrode wire 52 contacts the other inner electrode wire 52 due to being under the high-temperature environment for a long time.
The main cause of this problem is metal ion damage that a transition metal causes deterioration of an insulation (or polymer).
The metal ion damage is a phenomenon that a metal ion facilitates deterioration of a polymer (e.g., rubber, plastic) as an insulation, where a polymer deteriorates by a metal ion having catalysis (redox reaction) such that it facilitates auto-oxidation reaction of the polymer.
When the cord switch 50 is under the high-temperature environment for a long time, a hollow insulation 53 formed of a polymer deteriorates by the metal ion damage caused by a transition metal composing the outer electrode 51 such as copper etc. It is assumed that a low molecular component (antioxidant etc.) contained in the deteriorated hollow insulation 53 moves into a conductive rubber 54 of the inner electrode wire 52, the conductive rubber 54 swells to increase the volume resistivity of the conductive rubber 54. Here, “volume resistivity” is defined as an electrical resistance per unit volume.